Liquid crystal display devices (LCDs) are used for various apparatuses, such as notebook PCs, personal digital assistants, and digital cameras, which take advantage of the light weight, small thickness, reduced power consumption, and the like of the liquid crystal display devices. The display properties (luminance, color reproducibility, viewing angle characteristics, and the like) of the liquid crystal display devices have been much improved, and therefore, use of the liquid crystal display devices is spreading to desktop monitors in addition to conventional notebook PCs. In addition, by improving the color reproducibility of desktop LCD monitors, liquid crystal television sets having a big screen are being developed in these days. In general, however, improving the color reproducibility leads to a reduced transmittance of the color filter. Accordingly, a color filter having a high transmittance is desired.
While the liquid crystal display devices for desktop monitors require a color reproducibility of 50 to 65% in NTSC (National Television System Committee) ratio, liquid crystal television sets require a wider range of color reproducibility of 60 to 75% in NTSC. In addition to a wider range of color reproducibility, it is desired that, in the liquid crystal television set, each of the red, green, and blue colors has the same tone as that of conventional CRT television sets. The color properties of the conventional CRT television sets are substantially the same as in the EBU (European Broadcasting Union) standards in which the chromaticity coordinates of red, green, and blue in the XYZ colorimetric system are set at (0.640, 0.330), (0.290, 0.600), and (0.150, 0.060), respectively. Also, the color temperature of the white color displayed by a conventional CRT television set is set at about 9000 to 10000 K. This temperature is higher than the color temperature of the white color displayed by a liquid crystal display device for desktop monitors, which is set at 6000 to 7500 K. The liquid crystal television sets also need a high color temperature of the white color displayed by them.
Liquid crystal display devices, usually, display color images using a backlight source and a color filter in combination. Such a conventional combination of a backlight source and a color filter, however, does not lead to a color reproducibility, color tone, and a white color temperature satisfying the requirements for liquid crystal television sets. Specifically, the combination brings about problems in that (1) a small range of colors can be reproduced; (2) color tones in pixels is far away from the EBU standards; (3) the color temperature of the white color displayed thereby is low; and the like.
On the other hand, when a backlight source having a high color temperature (about 10000 K) is used for a liquid crystal display device in combination with the conventional color filter in order to increase the color temperature of the white color displayed thereby, the color tone takes on a blue tinge and thus fades. In particular, the red tone shifts to blue excessively in comparison with when a light source for desktop monitors is used. For example, when a backlight source having a high color temperature is used in combination with a color filter for desktop monitors, the red chromaticity, particularly the x value, is reduced to 0.590 to 0.615, and the red chromaticity is far from the EBU standard of (0.640, 0.330), which means the standard television color.
In order to prepare a color filter satisfying the EBU standards using a conventional backlight source for desktop monitors, it is required to increase the thickness of blue dots and the concentration of pigments and to improve the color purity. In this instance, unfortunately, the transmittance of the blue dots is greatly reduced. Consequently, the white color takes on a yellow tinge and thus the display characteristics deteriorate. Also, the white color temperature is low and thus the requirements for liquid crystal television sets are not satisfied.
In addition to the reduction of the transmittance, problems are caused in workability of the color filter and the shape of the dots. For example, when the thickness of the dots is increased to improve the color reproducibility but the concentration of pigments is not changed, the difference in height between the blue dots and the other dots is increased. This difference causes poor alignment of liquid crystal molecules and thus degrades the display quality. Also, even when the concentration of pigments is increased and the thickness of dots is reduced to improve the color reproducibility, workability is degraded and, for example, the dots are liable to crack.
Considering characteristics, such as transmittance, contrast, and chromaticity, pigments for the blue dots are selected from blue and violet colors, depending on the application. Exemplary pigments for the blue dots include Pigment Blue PB-6, PB-15 (15:1, 15:2, 15:3, 15:4, 15:6), PB-21, PB-22, PB-28, PB-60, and PB-64 and Pigment Violet PV-14, PV-19, PV-23, PV-29, PV-32, PV-33, PV-36, PV-37, PV-38, PV-40, and PV-50. In practice, some pigments having excellent characteristics are selected from these pigments. Blue pigments particularly with a phthalocyanine skeleton have excellent light stability and heat resistance, and therefore, they are suitable for use in color filters. When adjustment of the chromaticity is required, dioxazine violet is added to a blue pigment having a phthalocyanine skeleton. However, it is know that the transmittance is reduced in this instance (Japanese Unexamined Patent Application Publication Nos. 9-95638 and 9-197663).
As described above, the conventional combination of the back light and the color filter does not satisfy all the color characteristics (color reproducibility, color tone, and color temperature of the white color displayed), which are the requirements for liquid crystal display devices capable of reproducing a wide range of colors, and particularly for liquid crystal television sets.
Also, preparing a color filter having an excellent color reproducibility using a conventional blue paste causes problems such as transmittance shortage and poor alignment of liquid crystal molecules, and therefore a liquid crystal display device capable of providing high-quality images has not yet been achieved.